<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
        div{
            width: 2000px;
            height: 800px;
            /*a是alpha的缩写 透明度*/
            background: rgba(0,0,0,0.3);

        }
    </style>
</head>
<body>
<div>
aaaaaaaaaaaa
</div>
</body>
</html>